1. Field of the Invention
The invention relates generally to screening plants, which use vibratory screens of varying meshes to separate matter poured onto the screens, and more particularly relates to a displaceable eccentric that decreases unexpected, violent vibrations during starting and stopping of the vibratory screen of a screening plant.
2. Description of the Related Art
Conventional screening plants have been in use for some time. Such machines are used to separate particulate materials, which can be defined broadly as any material made up of a plurality of pieces of random size and shape, such as road construction debris, gravel, soil, sand and recyclables. Examples of screening plants are shown in many U.S. Patents, such as U.S. Pat. No. 6,000,553 to Cohen et al., U.S. Pat. No. 5,106,490 to McDonald and U.S. Pat. No. 4,923,597 to Anderson et al. Some screening plants are portable, permitting them to be transported to the location where excavation, mining or construction takes place.
Conventional screening plants ordinarily include an inclined, wide upper screen onto which material is poured either directly from a loading vehicle or by means of a conveyor. The upper screen vibrates, causing pieces of matter that are larger than the apertures of the screen to slide down its inclined surface onto a pile of larger pieces of matter that collect on one side of the machine. Matter that is smaller than the apertures in the upper screen drops through the apertures, typically onto a second angled screen with still smaller apertures, to be separated further. There can be numerous angled screens of various aperture sizes.
The drive mechanism for most screening plants includes an electric motor or an internal combustion engine that drives a pump for pressurizing hydraulic fluid. An example of such a mechanism is disclosed in U.S. Pat. No. 4,237,000 to Read. The hydraulic fluid is pumped to a hydraulic motor that rotates a driveshaft. The driveshaft extends through a screen box, which is a stack of similarly angled, parallel screens with progressively smaller apertures on each lower screen. The screens are attached to a rigid, peripheral frame. Fixed eccentric weights are mounted on opposite sides of the screen box to the driveshaft.
As the driveshaft rotates, the eccentric weights revolve about the axis of the driveshaft, causing the driveshaft and screen box to vibrate. The vibration causes the finer particulate matter, such as sand, to pass through the lowest screen layer. This finer particulate matter is often conveyed by an elevating conveyor from beneath the screen box to a pile spaced from the machine.
Problems arise from the use of conventional screening plants due to the vibration of the screens. The screens are vibrated, as described above, by rotating eccentric weights about a driveshaft axis. The conventional eccentric weights are massive plates with centers of gravity offset from the axis of the driveshaft, much like a crankshaft on an automobile engine. In order to begin rotating the driveshaft, the weights must be xe2x80x9clifted overxe2x80x9d the driveshaft by a substantial torque applied to the driveshaft. However, once the driveshaft is rotating at operating speed, the torque needed to keep it going is much lower.
The conventional internal combustion engine and hydraulic motor combination provides the needed torque for startup and operation, but is expensive and complex. A drive system that can only provide the small torque needed at operating speed does not have enough torque to start the driveshaft rotating because the difference between the startup torque and the operating torque can be very substantial.
Additionally, the fixed eccentric weights that cause the desired vibration of the screen box at operating speed can cause the screen box to shake violently at speeds less than operating speed. However, the machine must operate at speeds less than operating speed, such as during a warm-up period, and when the machine speed is being increased and decreased during startup and shutdown. Thus, during warm-up, startup and shutoff of the screening plant the screen box can vibrate in an undesirable manner that is potentially destructive to the screening plant and gives the screening plant the appearance that it is malfunctioning.
Therefore, the need exists for an apparatus that causes the screen box to vibrate at operating speed, but does not expose the screening machine to damage at other speeds and can operate with a less powerful drive system.
The invention is a vibration-control eccentric weight system for the vibratory screen of a screening plant. The screen screens particulate matter positioned thereon. The apparatus reduces or eliminates the violent vibrations of the screen during warm-up, startup and slowdown of the screening plant. The screening plant also has a driveshaft rotatably mounted to the screen and drivingly linked to a motor.
The eccentric weight system includes a plate rigidly mounted to the driveshaft. Another part of the eccentric weight system is a radially displaceable weight, which has a finger mounted within a radial slot formed in the plate. At least one bias, which is preferably a set of coil springs, is mounted to the plate and the weight for biasing the weight""s center of gravity toward close proximity to, but still spaced from, the driveshaft""s axis of rotation.
During no and slow rotation of the driveshaft, the eccentric weight system does not serve as an eccentric to any significant degree because the center of gravity of the eccentric weight system, which includes the weight, the plate and the springs, is aligned substantially along the driveshaft""s axis of rotation. This alignment is not perfect, and some eccentricity exists. The center of gravity of the weight, however, is spaced from the axis of the driveshaft. As the driveshaft rotates more rapidly, centrifugal force overcomes the bias of the spring and the weight is displaced radially outwardly. As the weight moves radially outwardly, its center of gravity is moved farther away from the axis of rotation of the driveshaft. This causes the eccentric weight system to serve more as an eccentric, causing the driveshaft, and the connected screen, to vibrate. As the driveshaft rotates more rapidly, the weight moves radially outwardly a greater distance and is more eccentric.